A transformer typically includes two or more inductively coupled windings that effect the transfer of electric energy from one circuit to another with a change in voltage, current, phase, or other electric characteristic. Transformers are used in many different electrical devices. For example, transformers are used in modern circuit breaker devices for sensing current in an electrical distribution circuit and providing a signal indicative of the sensed current to electronic circuitry, known as a trip unit, housed in the circuit breaker.
In modern circuit breaker devices, the transformer typically includes two multi-turn, secondary windings. One secondary winding is disposed around a top of the core and the other secondary winding disposed around the bottom of the core. Each of the secondary windings is electrically connected to the circuit breaker""s electronic trip unit. The transformer core is a toroidal, rectangular, or square shaped structure with an aperture disposed through its center. The primary winding is a primary conductor that extends through the aperture of the core. The primary conductor is electrically connected in series between a current carrying strap within the circuit breaker and a load conductor of the electrical distribution circuit. The primary conductor is a cast metal structure configured to support the core and the secondary windings.
In a circuit breaking device, the primary conductor is subjected to a very wide range of current within the operating range of the circuit breaking device. During quiescent operation, current through the primary conductor can be equal to a rated current of the circuit breaker, and during short circuit fault conditions the current through the primary conductor can exceed sixteen times (16xc3x97) the rated current of the circuit breaker. The transformer is designed to operate over this entire range. Design consideration for the transformer include: current measurement accuracy, temperature increase, are, and cost.
The current measurement accuracy of the transformer is dependent on the transformer""s ability to maintain a substantially linear relationship between flux intensity and flux density in the core throughout most of the operating current range (e.g., from 1xc3x97 to 16xc3x97 the rated current of the circuit breaker). To this end, the transformer is designed such that the core does not become saturated with magnetic flux at any point throughout the operating current range. Once the core becomes saturated, the linear relationship between flux intensity and flux no longer exists.
The physical placement of the primary conductor within the aperture of the core affects the point at which the core becomes saturated. As a result, it is desirable to center the primary conductor along the centroidal axis of the aperture of the core. However, due to space limitations in the circuit breaker housing, it is not always possible to place the primary conductor in the center of the aperture.
Where the primary conductor cannot be placed in the center of the aperture, transformers of the prior art have been designed with an increase in the size of the core in the section closest to the primary conductor. The additional material prevents magnetic saturation of the core in this section. Problematically, however, the increase in the size of the core is often times constrained by physical space limitations. In addition, the material added to the core increases the cost of the core.
In addition to being accurate over the operating current range, the transformer should not exceed predetermined temperature limits at any operating current within this range. For example, transformers should not exceed the temperature limits set by Underwriter""s Laboratories (UL) Section 489, which requires that the temperature of the transformer not exceed fifty degrees Celsius over ambient temperature.
The above discussed and other drawbacks and deficiencies of the prior art are overcome or alleviated by a transformer including: a core having an aperture formed therein, the aperture having a centroidal axis; and a conductive bar extending through the core. The conductive bar includes: a first surface extending generally parallel to the centroidal axis, a second surface opposite the first surface and extending generally parallel to the centroidal axis, the first surface being closer than the second surface to said centroidal axis, and means for diverting electrical current flowing through the conductive bar towards said first surface.
The above-discussed and other features and advantages of the present invention will be appreciated and understood by those skilled in the art from the following detailed description and drawings.